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Chelmsford Amateur Radio Society Intermediate Course (4) Receivers

Chelmsford Amateur Radio Society Intermediate Course (4) Receivers. Receivers. Summary Block diagrams of receivers The Superhet receiver Detection of AM, FM, and SSB How a Diode AM Detector works Use of BFO and CIO AGC. Tuned circuit. Diode detector. Earphone. Crystal Receiver.

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Chelmsford Amateur Radio Society Intermediate Course (4) Receivers

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  1. Chelmsford Amateur Radio Society Intermediate Course (4) Receivers

  2. Receivers Summary • Block diagrams of receivers • The Superhet receiver • Detection of AM, FM, and SSB • How a Diode AM Detector works • Use of BFO and CIO • AGC

  3. Tuned circuit Diode detector Earphone Crystal Receiver • Block diagram • Single tuned circuit; poor selectivity • No gain; poor sensitivity • Requires large aerial and earth; only receives strong signals

  4. Antenna D1 C1 AA119 100pF X1 L1 C2 C3 R1 Coil Crystal Earphone 500pF 100k 100pF RF Earth Crystal Receiver Circuit • L1, C1 - Tuned circuit – selects signal • D1 – Detector diode – demodulates • C3, R1 – Low-pass filter for audio

  5. RF amplifier Demodulator AF amplifier BFO TRF Receiver • Block diagram • RF amplifier gain increases sensitivity • One or more tuned circuits • All the gain is at one frequency – feedback is a problem • AF amplifier provides more power for loudspeakers

  6. Mixer IF amplifier Demodulator AF amplifier Local oscillator BFO Superhet Receiver • Block diagram • Mixer changes variable RF frequency to fixed IF frequency • IF amplifier provides selectivity with several tuned circuits • Fixed IF can use non-tunable crystal or ceramic filters

  7. ~ Mixer 145MHz–123.6MHz=21.4MHz IF frequency RF 145MHz ~ LO 123.6MHz Mixer as a Converter • Mixer may be used a frequency converter • Changes the selected RF frequency to the IF frequency using a tunable LO signal. • Mixers have spurious responses – image frequency, half the RF… • LO can be above or below the RF • IF can be above or below the RF Image frequency is 123.6MHz-21.4MHz=102.2MHz

  8. AM Envelope Detection • Demodulation of a modulated audio signal • Detector output follows envelope of RF • Otherwise known as an “envelope detector” Envelope Modulated RF Demodulated Audio

  9. Output Voltage D1 Time Demodulated AF Output RF Input C3 R1 100k 100pF Input Diode Detector Circuit • Diode D1 rectifies AC into DC • C1/R1 is a lowpass filter - filters out the RF • D1 conducts on positive half-cycle only

  10. 2 D1 cathode output 1 Diode Voltage / V 0 D1 anode input -1 -2 Anode Cathode 800 600 Diode conducts on peaks only Diode Current / µA 400 D1 current 200 0 Time/µSecs 0 0.5 1 1.5 2 2.5 3 3.5 4 Diode Detector Operation • Volts and Current in the diode detector (SPICE Simulation)

  11. + Carrier Wave Envelope Detector BFO Beat Note CW Demodulation - BFO • BFO = Beat Frequency Oscillator • Slightly offset BFO added to carrier to generate the “beat note” …the origins of the “BFO” !

  12. IF Sideband Filter IF Amp Product Detector AF Amp ~ CIO = Carrier Insertion Oscillator CIO SSB Demodulation • SSB filter selects only the wanted sideband • Product detector mixes to baseband • CIO is at the frequency where carrier would have been • Product detector is a balanced mixer

  13. Amplitude Lower Sideband Upper Sideband Frequency MHz IF Filter Mixer Amplitude Amplitude Upper Sideband Upper Sideband Frequency MHz Frequency USB Demodulation • SSB demodulation is essentially mixing to baseband • Unwanted sideband may contain noise and other signals • Mixing with the carrier frequency is product detection

  14. Amplitude Lower Sideband Upper Sideband Frequency MHz IF Filter Mixer Amplitude Amplitude Lower Sideband Lower Sideband Frequency MHz Frequency LSB Demodulation • SSB demodulation is essentially mixing to baseband • Typically, IF filter is not moved; the local oscillators are offset. • When mixed down, the LSB spectrum becomes inverted.

  15. Demodulation Summary • CIO = Carrier Insertion Oscillator • A fixed local oscillator used to demodulate SSB • It reinserts the carrier that was removed in the transmitter • Wanted sideband is directly translated directly to Audio • BFO = Beat Frequency Oscillator • A slightly variable local oscillator used to demodulate CW • CW demodulated by envelope detector • BFO/Detectors can resolve SSB. A product detector can resolve CW • Two Issues • Input Signal Levels can vary greatly • Detection methods described so far are for Amplitude based modulations, not FM

  16. RF Amp Mixer IF Amp Detector AF Amp LO AGC Circuit AGC • AGC = Automatic Gain Control • AGC samples the signal strength at the detector and then feeds back a control signal to adjust the gain of the earlier stages • AGC keeps audio volume fairly constant if signals level vary

  17. Mixer IF amplifier Discriminator AF amplifier Local Oscillator Squelch circuit FM Receiver • Uses FM Discriminator to detect small frequency deviations • FM receivers use IF limiter amplifiers; no AGC needed • Squelch detects high-frequency noise and gates audio

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